An ex-ally and business partner of former Sen. Alfonse D’Amato slapped the power broker with a federal lawsuit Thursday alleging he and others swindled her out of at least $5 million in commissions — and tried using her as a “stalking horse” to deceive the feds into giving them lucrative contracts set aside for women-owned businesses.

Mary Arnold, a longtime DC lobbyist and Florida resident, is seeking $20 million in punitive damages and to recover at least $5 million she claims she is owed through a contract with Precision Discovery, a Manhattan-based litigation consulting firm run by the longtime New York politician and his brother, Armand D’Amato, the suit claims.

The Manhattan federal suit says Precision, which is also named as a defendant, created an entity known as AEDDiscovery LLC while trying to score government contracts worth hundreds of millions of dollars to provide electronic document retrieval services for use in court.

It claims the D’Amato brothers and another co-defendant, Precision CEO Jerry Barbanel, insisted Arnold “assume the role of a figurehead” at AEDDiscovery — even though she would have “no management control” — so the male-run firm could take advantage of favorable treatment given to women-owned small businesses.

The suit claims when Arnold refused to take part in the “scam” they retaliated against her by depriving her of at least $5 million in lobbying commissions she earned helping Precision land a lucrative contract with AT&T.

“She has a long, credible history in Washington, and she did not want to be involved in anything that would mislead the government or harm her reputation,” her lawyer Paul Batista told The Post.

Arnold was recruited in 2009 by Armand D’Amato at the request of his powerhouse brother, the suit says.

Arnold remained under contract with Precision until August 2013, the suit says.

According to company filings, Armand D’Amato is listed as board director of Precision, but Al D’Amato is not listed as having a position in the firm.

The suit claims that the defendants’ plan was to make Arnold majority shareholder in name only for AEDDiscovery as it pursued the fed contracts. But she would have no power and see little — if any — of the profits.

Alfonse D’Amato in a email said he “categorically denies” the “meritless” suit’s “salacious” claims. He also said “we intend to defend ourselves against these outrageous accusations” and sue Arnold for libel and defamation of character.